A common requirement for electronic equipment is to have a visual indication on a control device. Such a control device could be a button, lever, knob, or similar, and the visual indication is often used to show the state of whatever is controlled—for example an electrical load being turned on or off, or a door open or closed.
In modern electronic equipment, the visual indication is readily provided by a light emitting diode (LED) which has the advantages of being small, cheap, and a long operating life. Multiple colours may be achieved by making use of several LEDs of different colour—these can even be mounted in the same physical package.
By separate dimming of each component colour, a dual-colour combination LED can be used to mix the two component colours to create additional colours, as perceived by a human observer. More recently, three colour combination LEDs have become available, and some of these include elemental components that emit in the colours Red, Green and Blue. By selective dimming of these components, any colour, including white, can be obtained.
In order to produce any resultant colour, an RGB combination LED needs each of the colour components to have a separate dimmable drive. Whilst LED dimming is straightforward using the well-established method of pulse-width modulation, such a typical combination LED will usually have at least 4 terminals: a common point and a drive input for each of the three colour elements.
Using a single three colour RGB combination LED with a common point and 3 drive inputs is relatively easy to drive to obtain any desired human-perceived colour. For example, a small micro-processor can use 3 separate pulse-width modulated outputs to drive such an indicator. An exemplary arrangement of this application is shown in prior art FIG. 1. In FIG. 1, there is shown microprocessor 10 driving two LEDs 30 and 30′. Each LED has respective colour components of red (31), green (32) and blue (33). Respective resistors 34, 35 and 36 are also provided to limit the current applied to the LED components.
In operation, microprocessor 10 generates and transmits control signals to each LED 30 and corresponding colour component 31, 32 and 33 to control the operation of each LED in accordance with a sequence of instructions programmed into microprocessor 10 as will be understood by the person skilled in the art.
As can be seen from FIG. 1, driving 2 LEDs requires 6 drive signals 11, 12, 13, 14,15 and 16, one for each LED component, or three for each LED in the case of a three-colour LED.
It will be appreciated however, that when many such combination LEDs need to be driven (irrespective of the number and nature of the coloured elements)—potentially all showing separate information (for example colour and brightness) the number of drive signals rapidly increases. For example to drive 10 such RGB combination LEDs would require 30 drive signals. As the number of multi-colour combination LEDs in a product increases, the complexity of wiring these to a controller device increases, as does the complexity of the controlling device. For example, a large number of wiring paths might be needed on a printed circuit board, and a large number of pulse width modulated outputs might be needed on a microprocessor.
LED controller devices are currently available that make use of serial data buses—the most notable being the Philips inter-integrated circuit (I2C) bus. Such LED controllers allow a 2-wire control output from a microprocessor to be connected to a LED driver IC, which in turn is connected to one or more combination LEDs. By sending appropriate commands to the I2C driver IC, a suitable colour and brightness can be obtained from the combination LED(s). If the combination LED contains red, green and blue elements, then by appropriate selection of brightness of the elements, any colour can be obtained.
Whilst being an improvement on directly connecting LEDs to a microprocessor, systems using these approaches have a number of deficiencies. These deficiencies include the high cost of the LED driver ICs, the limited addressing range of these bus systems, and the additional need to route power and/or ground separately to the LEDs and the driver IC's—increasing circuit board layout complexity and potentially cost, through the need for multi-layer PCBs. Whilst this limited addressing can be overcome by the use of I2C bridges, this adds extra devices and cost to such an arrangement.
It is therefore an object of the present invention to provide an alternative to existing LED control systems.